The present disclosure relates to prosthetic heart valves and, more specifically, to prosthetic heart valves having a collapsible stent frame.
Current collapsible prosthetic heart valve designs are for use within high-risk patients who may need a cardiac valve replacement, but who are not appropriate candidates for conventional open-chest, open-heart surgery. To address this problem, collapsible and re-expandable prosthetic heart valves have been developed that can be implanted transapically or percutaneously through the arterial system. However, such collapsible valves may have important clinical issues because of the nature of the patient's native stenotic leaflets that may not be resected as with the standard surgical practice of today. Additionally, patients with uneven calcification, bicuspid disease, and/or aortic insufficiency may not be treated well with the current collapsible prosthetic valve designs. The limitation of relying on evenly calcified leaflets has several issues, such as: (1) perivalvular leakage (PV leak), (2) valve migration, (3) mitral valve impingement, (4) conduction system disruption, etc., all of which can have adverse clinical outcomes. To reduce these adverse events, the optimal valve would seal and anchor to the cardiac tissue adequately without the need for excessive radial force that could harm nearby anatomy and physiology. An optimal solution may be to employ a stent that exerts a radial outward force just large enough to hold open the native stenotic/insufficient leaflets, and to use additional anchoring features more reliant on another anchoring methodology while reducing leaflet/stent stresses.
After multiple clinical valve failures during the late 1960's and early 1970's, a series of investigations on leaflet failure (e.g., dehiscence at the commissures) and stent post flexibility began and continue to be explored today. (Reis, R. L., et al., “The Flexible Stent: A New Concept in the Fabrication of Tissue Heart Valve Prostheses”, The Journal of Thoracic and Cardiovascular Surgery, 683-689, 1971.) In-vitro, animal, and clinical investigations showed that “a flexible stent greatly reduces stress on the valve,” which was as large as a 90% reduction of the closing stresses near the commissures when flexibility and coaptation area were maximized.
In more recent years, several groups have shown (e.g., via numerical computations) the importance of stent post flexibility during opening and closing phases to reduce leaflet stress and therefore tissue failure. (Christie, G. W., et al., “On Stress Reduction in Bioprosthetic Valve Leaflets by the Use of a Flexible Stent,” Journal of Cardiac Surgery, Vol. 6, No. 4, 476-481, 1991; Krucinski, S., et al., “Numerical Simulation of Leaflet Flexure in Bioprosthetic Valves Mounted on Rigid and Expansile Stents,” Journal of Biomechanics, Vol. 26, No. 8, 929-943, 1993.) In response to several rigid Ionescu-Shiley clinical valve failures in which the leaflets tore free at the commissures, Christie et al. (cited above) explored what would happen if a similar design was made with optimal flexibility. Stresses at the post tops were shown to be five times greater than at the belly of a leaflet. Thus, to optimize the design, the stent was made more flexible until the stresses in the leaflets were comparable to those in the leaflet belly. It was shown that a 0.2-0.3 mm deflection was all that was needed to make a significant reduction in stress, but that a deflection of approximately 1.1 mm would reduce the stress by up to 80%. Furthermore, it was explained that deflection beyond 1.1 mm was not only difficult to achieve with the available material and design, but did not result in additional stress reduction.
Krucinski et al. (also cited above) have shown that a 10% expansion (as may be the case during the opening phase of a Nitinol stent) may reduce sharp flexural stresses by up to 40% (e.g., “hooking”). This is likely due to the stent functioning in harmony with the patient's aortic root, or in other words, the commissures of the native valve moving outward during systole.
Although the above analyses and data may not be directly applicable to the collapsible valve designs detailed later in this specification, the basic understanding and theory about how pericardial tissue leaflets interact with a stent design as it functions are important to incorporate into any design where durability is paramount. It is possible that with good engineering design of the post and leaflet attachment, commissural dehiscence will not be a primary failure mechanism.